UNIT III STUDY GUIDE KEY
I. Vocabulary Check
1. X21. V
2. H22. II
3. W23. I
4. L24. JJ
5. F25. AA
6. P26. CC
7. Q27. G
8. J28. S
9. U29. O
10. EE30. R
11. Z31. HH
12. DD32. N
13. FF33. K
14. KK34. GG
15. A35. D
16. M36. E
17. Y37. LL
18. T38. BB
19. C
20. B
II. Photosynthesis
1. A – thylakoid – site of Light Rxn; converts light energy to chemical, electron energy
B – inner thylakoid space – location of build-up of H+ ions; created electrochemical gradient, driving force
for ATP synthase
C – stroma – site of Calvin Cycle; formation of glucose precursor
2. DNA & ribosomes; mitochondria
3. palisade mesophyll
4. 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
CO2 is reduced; H2O is oxidized
5. Water diffuses into the roots due to lower water potential inside roots, moves up plant due to lower water
potential in leaves through adhesion and cohesion
CO2diffuses into leaves through stomata due to concentration gradient
AN OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis consists of two pathways, the Light Dependent Reaction which occurs in the thylakoids and the Calvin Cycle, which takes place in the stroma of chloroplasts.
In the first pathway,light energy is converted to chemical energy in the form of ATP and NADPH. The first step involves Photosystem II. Light energy packaged in photons, primarily in the red and blue portions of the visible spectrum, is absorbed by antenna pigment molecules. The energy is passed along until it reaches the reaction center, a pair of chlorophyll a molecules known as P680. The reaction center chlorophyll molecules respond to this energy by losing 2 electrons to the primary electron acceptor. The electrons then move through an electron transport chain. As the electrons move through the ETC, energy is released and used to move H+ into the inner space (lumen) of the thylakoids. This creates an electrochemical gradient (also called the proton motive force) which is used to power the enzyme complex, ATP synthase. As H + ions pass through the enzyme complex and move into the stroma, an inorganic phosphate group is added to ADP, creating ATP. This process is known as photophosphorylation. The electrons originally lost from the reaction center are replaced by the splitting of water, producing O2 and H+ ions. In Photosystem I, as light energy is captured and transferred, a pair of chlorophyll a molecules, known as P700, are excited, causing the loss of 2 electrons. The two excited electrons are passed through a short electron transport chain ending with the reduction of NADP+ to NADPH. The electrons lost by the reaction center in this photosystem are replaced by electrons originally lost from P680.
There is an alternative pathway seen in some bacteria and plants which only utilizes photosystem I. This is a cyclic pathway in which electrons are simply recycled. Although ATP is created, there is no production of NADPH or O2.
The second part of photosynthesis is known as the Calvin Cycle. There are three phases in this cycle,
Carbon Fixation,Reduction Phase, and Regeneration Phase. First, 3 CO2 are added to 3 ribulose biphosphates, abbreviated as RuBP. This requires the action of the enzyme, rubisco. The resulting intermediate splits, and using energy provided by ATP, is then reduced forming G3P, and oxidizing NADPH to NADP+ . One molecule of G3P leaves the cycle to form glucose and other carbohydrates. This is known as the Reduction Phase. Finally, in the last phase, RuBP is regenerated, requiring additional ATP.
Synthesis of glucose requires 6 turns of the Calvin cycle. In addition other carbohydrates can be synthesized including cellulose for plant cell walls, starch for glucose storage, and the disaccharide, sucrose (composed of glucose and fructose monomers) often used for transport in the plant.
III. Cellular Respiration
1. A – cristae – location of electron transport chain; allows for creation of proton motive force (electrochemical
gradient)
B – matrix – location of citric acid cycle; generates ATP, NADH, FADH2
C – intermembrane space – site into which H+ ionsare pumped using energy from “falling” electrons in ETC
2. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ~38 ATP
C6H12O6 - oxidized
O2 – reduced
The first step in cellular respiration is glycolysis. This occurs in the cytosol of cells and is an anaerobic pathway. It is divided into two phases, the energy-investment phase and the energy pay-off phase. In the first phase, 2 ATP are required to provide the energy to split glucose into 2 G3P molecules. In the 2nd phase, these molecules are oxidized, producing 2 pyruvate, 2 NADH, and 4 ATP for a net gain of 2 ATP.
If oxygen is present, the pyruvate formed from glycolysis moves into the mitochondria. An intermediate step takes place prior to the citric acid cycle. First, a carboxyl group is given off as CO2. The remaining 2-C molecule is oxidized, reducing NAD+ to NADH. Finally, the oxidized 2-C molecule attaches to an enzyme complex to form acetyl CoA.
This complex enters the citric acid cycle. A series of redox reactions take place, producing 6 NADH and 2 FADH2,. In addition, carboxyl groups are removed, releasing CO2 and 2 ATP are produced through substrate-level phosphorylation. The reduced electron carriers formed in the citric acid cycle move to the electron transport chain and the electrons are “dropped” from one molecule to another, with each successive molecule more electronegative than the one before it. The ultimate electron acceptor is oxygen which is reduced to form water. As the electrons fall, their energy is used to drive H+ from the matrix to the intermembrane space, creating an electrochemical gradient. This gradient, also known as the proton motive force, powers the enzyme complex, ATP synthase, and ADP is phosphorylated to produce ATP. Each NADH produces approximately 2.5 ATP and each FADH2 produces about 1.5 ATP. There are 2 NADH produced in glycolysis, 2 NADH formed in the intermediate step, and 6 NADH & 2FADH2formed in the citric acid cycle so there is enough electron energy to produce a total of ~28 ATP. The ATP produced through oxidative phosphorylation is added to the 2 ATP from glycolysis and the 2 ATP from the citric acid cycle for a total of ~32 ATP produced per molecule of glucose in cellular respiration.
IV. A COMPARISON OF CELLULAR RESPIRATION & PHOTOSYNTHESIS
Characteristic / Cellular Respiration / Photosynthesis1. Type of metabolic reaction / Catabolic, exergonic / Anabolic, endergonic
2. Purpose of Pathway / Convert chemical energy to ATP / Convert light energy to chemical energy
3. Reactants required / C6H12O6 + O2 / CO2 + H2O + energy
4. End products / CO2 + H2O + energy / C6H12O6 + O2
5. Occurs in cells of what organisms? / Virtually all actively-metabolizing cells require some type of energy pathway / Cells that contain chlorophyll
6. Site(s) involved in eukaryotic cells / Cytosol, mitochondria / Chloroplasts
7. Site(s) involved in prokaryotic cells / Cytosol, cell membrane / Cytosol, cell membrane
8. Mechanism for ATP production / Substrate-level and oxidative phosphorylation / Photophosphorylation
9. Electron Transport Carrier Involved / NAD+, FAD / NADP+
10. Location of ETC / Mitochondrial inner-membrane / Thylakoid membrane
11. Source of Electrons for ETC / Glucose / Water
12. Terminal Electron Acceptor / Oxygen / NADP+